IEEE 802.11s is a draft specification for providing a means to form a mesh wireless backhaul with IEEE 802.11 WLAN technology. Mesh networks are also known as multi-hop networks, since data packets may be relayed more than once in order to reach their destination. This presents a different paradigm as compared to the original WLAN standard, which addresses only star topologies for stations (STAs) to be connected to an access point (AP) effectively using single hop communications through a basic service set (BSS).
IEEE 802.11s only addresses network nodes that form a mesh network and the WLAN mesh operation in the backhaul that is transparent to all STAs. This means that, similar to legacy IEEE 802.11 WLAN, STAs still connect to an AP, (i.e., mesh AP having a mesh capability), through a BSS. The mesh AP interfaces on its backhaul side to other mesh points which forward and route traffic through the mesh network to a destination. The destination may be a mesh portal which routes the traffic to the external network or may be another mesh AP attached to the mesh network. By choosing this approach, even legacy STAs may still operate in a mesh-enabled WLAN. The communication between STAs and a mesh AP in a BSS is completely independent from the mesh network. The STAs are unaware of the presence of the mesh network in the backhaul.
The IEEE 802.11s WLAN mesh standard has been designed under the assumption that legacy IEEE 802.11a/b/g radio interface may be implemented on the mesh points. The IEEE 802.11s standard is mostly radio interface agnostic. For example, routing and forwarding of data packets are not dependent on the particularities of an IEEE 802.11a/b/g radio interfaces, (such as modulation scheme or channel coding).
IEEE 802.11s also allows different ways for simultaneous multi-channel operation. One way to implement multi-channel operation is to use multiple IEEE 802.11 radio devices on a mesh point in order to increase the available data throughput capability. Another possibility is to use a single radio device, (so-called common channel framework (CCF)), for more than one channel.
IEEE 802.11n specification is another specification for providing a high throughput (HT) WLAN. Some of the IEEE 802.11n throughput-enhancing features are aggregation, enhanced block acknowledgement (BA), reverse direction grant, power save multiple poll (PSMP), and operational bandwidth. In IEEE 802.11n, a data rate is increased by annexing or bonding two adjacent channels. The data rate increase is also achieved by using several more data tones with 802.11 40 MHz operation relative to 2×20 MHz channel occupancy with 802.11a/g. However, not all IEEE 802.11n devices may support 40 MHz operation and, therefore, the transition of operation from 20 MHz to 40 MHz should be managed efficiently. In order to achieve this, the IEEE 802.11n standard provides some channel management mechanisms.
In IEEE 802.11n, three operating modes are allowed according to bandwidth and BSS capability: 20 MHz operation, 20/40 MHz operation and Phased Coexistence Operation (PCO). Each of these modes has associated rules of operation. In a 20 MHz operation, all STAs will operate only in a 20 MHz mode whether or not the STAs are 20 MHz or 20/40 MHz capable. In a 20/40 MHz operation, STAs choose the bandwidth by using a transmission channel width action message. In addition, a 40 MHz device will protect its transmission with legacy control frames such as request-to-send (RTS) or clear-to-send (CTS) frames, if the AP of its BSS indicates that there are 20 MHz and/or legacy STAs in the BSS. In the PCO mode, which is an optional mechanism, the BSS alternates between 20 MHz and 40 MHz modes.
Although the IEEE 802.11s WLAN mesh standard attempts to remain radio agnostic to the largest extent possible, integration of an IEEE 802.11n high-throughput radio, instead of an 802.11a/b/g, still poses several problems. For example, unlike previous IEEE 802.11a/b/g systems that operate only in 20 MHz bandwidth, IEEE 802.11n operates in both 20 MHz and 40 MHz bandwidths.
With Enhanced Distributed Channel Access (EDCA)-based mesh channel access mode, when a mesh point contends and gets an access to the channel, mesh points on a particular link or in a neighborhood have to agree either before the channel access or during the channel access about the specifics of the channelization scheme to be used, (i.e., 20 MHz vs. 40 MHz). Moreover, IEEE 802.11n uses a slightly altered sub-carrier configuration when using full 40 MHz mode, (i.e., using a higher number of data tones when operating in 40 MHz mode compared to dual channel 2×20 MHz 802.11a radios). Dual 2×20 MHz channel operation is also possible for coexistence with legacy radios. Because current IEEE 802.11s technology only allows orthogonal frequency division multiplexing (OFDM) parameters to be communicated to advertise current channel identification to neighbor MPs, the use of IEEE 802.11n radios in an IEEE 802.11s WLAN mesh networks is severely limited due to the limitation with legacy 20 MHz mode even if an IEEE 802.11n radio is used.
Another problem with the current IEEE 802.11s WLAN mesh networks is the set-up and configuration of a particular mesh link, a neighborhood in the mesh or the entire mesh network with respect to high-throughput channelization modes and configurations to be used. For example, it is currently not possible to prevent or allow the use of 40 MHz access in either version (full 40 MHz or 2×20 MHz) on a particular link, in a mesh neighborhood, or for the entire mesh. This is a limitation with the current IEEE 802.11s technology in the sense that it constitutes an obstacle towards efficient use of 802.11n radios and all the proposed IEEE 802.11n enhancements with WLAN mesh technology.
Therefore, it would be desirable to have a scheme to overcome the above shortcomings and allow efficient integration of IEEE 802.11n radios into IEEE 802.11s WLAN mesh networks.